Oil shale is a naturally-occurring, shale-like rock which contains an organic component, usually referred to as kerogen, that upon heating releases volatile hydrocarbons recoverable as shale oil. Following pyrolysis of the kerogen, a residual carbonaceous material remains along with the mineral component that may be burned to yield heat for the pyrolysis of fresh oil shale. The hot mineral residue that remains after combustion of the carbonaceous component is recycled in some retorting schemes as "heat transfer material", i.e., the hot burned shale from the combustion is mixed with fresh oil shale and the heat provided is used for pyrolyzing the fresh shale. Although retorting schemes using recycled shale as heat transfer material have significant advantages over other retorting systems, the burned oil shale is not an ideal material for this purpose.
During pyrolysis of the kerogen and combustion of the remaining carbonaceous residue, the inorganic matrix undergoes both chemical and physical changes. It has been observed that the use of recycled burned oil shale particles as heat transfer solids can lead to significant yield reductions of the shale oil product. These losses, in some instances, can run as high as 15% to 20% Controlling these losses would be important in any commercial scheme for recovering shale oil.